Vapor cell and method for making same

ABSTRACT

Vapor cells and methods for making the same are presented, in which a cell cavity is completely filled with aqueous alkali metal azide solution and the solution is dried at a controlled evaporation rate to substantially maintain edge contact pinning at an interface with the cavity sidewall to promote preferential evaporation in the center and outward capillary flow from an unpinned air-fluid interface toward the sidewall to form crystallized alkali metal material at the sidewall while inhibiting drying of dispersed aqueous solution on a transparent cavity bottom to provide substantially unrestricted passage of light through the cavity for atomic clock and other applications.

REFERENCE TO RELATED APPLICATIONS

The following U.S. Patent Applications are referenced: Ser. No.14/309,966 filed Jun. 20, 2014; Ser. No. 14/309,223 filed Jun. 19, 2014;Ser. No. 14/319,310 filed Jun. 30, 2014; and Ser. No. 14/319,696 filedJun. 30, 2014.

FIELD OF THE INVENTION

The present disclosure relates to vapor cells and more particularly toimproved vapor cells and fabrication methods.

BACKGROUND

Vapor cells or atomic vapor cells are used in the construction of atomicclocks, magnetometers, and other devices, and allow chip-scalestructures with significant power consumption advantages for use inportable battery-powered devices over conventional atomic clocks.Ideally, an alkali metal gas such as Cesium (Cs) or Rubidium (Rb) isprovided within a sealed cavity and a light source optically excites thealkali metal, with the frequency of electronic transition of an alkaliatom having a single electron in the outer shell being used as anabsolute frequency reference for generating highly accurate clocksignals. Optical interrogation at different frequencies within afrequency band can be used to identify a transition frequency throughabsorption spectrum detection to provide an absolute frequency referencefor a clock. Proper operation for atomic clock or other applications isfacilitated by a clear unobstructed optical path through the vapor cell.One method of adding a controlled amount of Cs to a vapor cell is tointroduce aqueous Cesium Azide (CsN₃) into the cell and then allow thewater to evaporate leaving solid CsN₃ residue. After the vapor cell issealed, UV photolysis is carried out to dissociate the CsN₃ residue intoCs and N2. However, conventional vapor cell fabrication techniquessuffer from formation of residual alkali metal azide on the lower cellwindow, causing undesirable absorption and/or scattering of light duringcell operation. Such CsN₃ or other alkali metal solids, moreover, cannotbe removed from the vapor cell window by post-processing. Accordingly, aneed remains for vapor cells with unobstructed optical paths forefficient identification of the alkali metal electronic transmissionfrequency for atomic clocks and other applications.

SUMMARY

The present disclosure provides fabrication processes or methods andvapor cell apparatus by which undesirable optical obstructions in thelower cell window can be mitigated or avoided through substantiallycomplete filling of a cell cavity with aqueous alkali metal solution anddrying the solution in a controlled environment to substantiallymaintain edge contact pinning of the solution at the cavity sidewall forenhanced evaporation in the unpinned center and outward capillarysolution flow toward the sidewall to form crystallized alkali metalmaterial at the sidewall while inhibiting drying of dispersed aqueoussolution on the lower cell window. The resulting cell structure providesa completely or substantially unrestricted light channel through thecavity for operation in atomic clock and other applications.

In accordance with one or more aspects of the disclosure, a method isprovided for fabricating a vapor cell, in which a cavity is formedbetween first and second sides of a substrate, and a lower window isbonded to the second side to form a cavity bottom. The cavity is filledwith an aqueous solution comprising an alkali metal material, such asCsN₃ in one embodiment, with the solution extending to a contact linesubstantially at the top of the cavity sidewall or sidewalls. The methodfurther includes drying the aqueous solution at a controlled temperatureand humidity to substantially maintain edge contact pinning at a pinnedfluid interface between the solution and the sidewall to promotepreferential evaporation near the center of an unpinned air-fluidinterface, along with outward capillary fluid flow toward the pinnedfluid interface to promote drying of dispersed aqueous solution andformation of crystallized alkali metal material at or near the sidewalland to inhibit drying of dispersed aqueous solution on the cavitybottom. The method further includes bonding an upper window to the firstsubstrate side to seal the cavity and exposing the crystallized alkalimetal material to UV photolysis to form an alkali metal gas in thesealed cavity.

Further aspects of the disclosure provide a vapor cell structurecomprised of a substrate with first and second sides and a cavitydefined by one or sidewalls extending between the substrate sides, aswell as a crystallized alkali metal material formed on all or a portionof the sidewall or sidewalls. The cell further includes a lower windowforming a transparent cavity bottom substantially free of crystallizedalkali metal material, an upper window to seal the cavity and form atransparent cavity top, and an alkali metal gas in the sealed cavity.

DESCRIPTION OF THE VIEWS OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrated examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure will be setforth in the following detailed description when considered inconjunction with the drawings, in which:

FIG. 1 is a top plan view illustrating a Cesium vapor cell in accordancewith one or more aspects of the present disclosure;

FIG. 2 is a partial sectional side elevation view of the Cesium vaporcell taken along line 2-2 in FIG. 1;

FIG. 3 is a flow diagram illustrating a method for fabricating a vaporcell in accordance with further aspects of the disclosure;

FIG. 4 is a partial side elevation view illustrating top and bottompolishing of a silicon substrate;

FIG. 5 is a partial side elevation view illustrating masked wet etchingto form a cavity with angled sidewalls extending between the top andbottom sides of the substrate of FIG. 4;

FIG. 6 is a partial side elevation view illustrating bonding of a lowerwindow to the lower side of the substrate of FIG. 5 to form a cavitybottom;

FIGS. 7A and 7B are partial side elevation and top plan viewsillustrating complete filling of the cavity with an alkali metal aqueoussolution;

FIGS. 8A and 8B are partial side elevation and top plan viewsillustrating initiation of a drying process at a controlled temperatureand relative humidity to provide edge contact pinning of the alkalimetal aqueous solution at the cavity sidewalls and preferentialevaporation at the center;

FIGS. 9A and 9B are partial side elevation and top plan viewsillustrating continuation of the drying process with continued edgecontact pinning at the sidewalls and outward capillary flow of theaqueous solution from the center toward the sidewalls;

FIGS. 10A and 10B are partial side elevation and top plan viewsillustrating further evaporation of the aqueous solution;

FIGS. 11A and 11B are partial side elevation and top plan viewsillustrating continuation of the drying process with a portion of thebottom cavity window exposed and continued outward capillary flow of theaqueous solution toward the sidewalls;

FIGS. 12A and 12B are partial side elevation and top plan viewsillustrating completion of the drying process with substantially all ofthe crystallized alkali metal material transferred to the cavitysidewalls;

FIGS. 13A and 13B are partial sectional side elevation and top planviews illustrating bonding of an upper window to the top side of thesubstrate to seal the cavity; and

FIGS. 14A and 14B are partial sectional side elevation and top planviews illustrating exposure of the crystallized alkali metal material tophotolysis to form an alkali metal gas in the sealed cavity.

DETAILED DESCRIPTION

One or more embodiments or implementations are hereinafter described inconjunction with the drawings, wherein like reference numerals are usedto refer to like elements throughout, and wherein the various featuresare not necessarily drawn to scale.

Referring initially to FIGS. 1 and 2, the present disclosure providesfabrication techniques and vapor cell structures 100 which may beemployed in a variety of applications, including without limitationatomic clocks, magnetometers and the like. FIGS. 1 and 2 illustrate aCesium vapor cell embodiment 100 having a single cavity 103 defined bysidewalls 104 formed in a silicon substrate 102 between atomicallypolished first and second (top and bottom) sides 102 a and 102 b (asshown in the sectional side view of FIG. 2 taken along line 2-2 in FIG.1). The cell cavity 103 is further defined by a transparent first(lower) silicon glass window 106 engaging and bonded to the second side102 b to form a cavity bottom 103 b, and a transparent second (upper)silicon glass window 106 engaging and bonded to the first side 102 a ofthe substrate 102 to seal the cavity 103. The cavity 103 is defined by asingle opening in the substrate 102 with four angled sidewalls 104extending from an opening in the top side 102 a to an opening in thebottom side 102 b to form a trapezoidal cavity 103 in the illustratedembodiment with a rectangular shape in the X-Y plane providing anoptical path along the vertical Z direction as illustrated in FIG. 2.Other embodiments are possible having one or more sidewalls 104, such asa circular cavity with a single sidewall 104, and other cavity shapesmay be provided in different embodiments. In the illustrated embodiment,moreover, the sidewalls 104 are at a non-zero angle θ relative to avertical Z axis in FIG. 2 extending between the first and second sides102 a and 102 b. For example, the angle θ in the embodiment of FIG. 2 isapproximately 54.74°, although angled sidewalls 104 and this particularangle value are not strict requirements of all embodiments.

In addition, as discussed further below, a crystallized alkali metalmaterial 105 is formed on at least a portion of the sidewalls 104, andthe lower window 106 forming the cavity bottom 103 b is substantiallyfree of crystallized alkali metal material 105. The cell 100, moreover,includes an alkali metal gas in the sealed cavity 103. In theillustrated embodiment, Cesium metal gas is provided in the cavity 103as detailed further below. In other possible embodiments, other alkalimetal gas may be provided in the cavity 103, including withoutlimitation rubidium, potassium, etc. Provision of the transparent upperand lower windows 108 and 106, and the provision of the lower window 106substantially free of crystallized alkali metal material advantageouslyprovides a generally unobstructed optical channel for laser or otheroptical excitation or interaction of the alkali metal gas within thecavity 103 without undesirable optical obstructions, and the vapor cell100 of FIGS. 1 and 2 is thus ideal for atomic clock, magnetometer, andother applications. Furthermore, the cell 100 requires only a singlecavity 103 and is ideal for applications requiring low manufacturingcost and small device size.

In the illustrated embodiment, moreover, the sidewalls 104 have asurface texture that is significantly rougher than the polished surfacetexture of the first or top side 102 a of the substrate 102.Furthermore, the sidewall texture is also significantly rougher thanthree upper surface of the lower silicon glass window 106 forming thecavity bottom 103 b. As discussed further below, this rough sidewalltexturing advantageously facilitates edge contact pinning of CesiumAzide or other alkali metal aqueous solution at or near the top of thesidewalls 104 when drying begins, so as to facilitate provision of agenerally obstruction-free transparent cavity bottom 103 b. In addition,as shown in FIGS. 1 and 2, the cell 100 is a single-cavity configurationwith only one cavity 103, which is separated by the sidewalls 104 fromany other cavity formed in the substrate 102 during fabrication, unlikedual cavity vapor cell configurations in which different materials arereceived in different cavities with a channel providing fluid connectionof the two cavities. Accordingly, the disclosed cell design 100 is acompact device ideal for portable device applications.

Referring now to FIGS. 3-14B, FIG. 3 illustrates an example fabricationprocess or method 200 for fabricating a vapor cell, and FIGS. 4-14Billustrate the cell 100 of FIGS. 1 and 2 undergoing various stages offabrication according to the method 200. Although the method 200 isillustrated and described in the form of a series of acts or events, thevarious methods of the disclosure are not limited by the illustratedordering of such acts or events except as specifically set forth herein.In this regard, except as specifically provided hereinafter, some actsor events may occur in different order and/or concurrently with otheracts or events apart from those illustrated and described herein, andnot all illustrated steps may be required to implement a process ormethod in accordance with the present disclosure. The illustratedprocess 200 begins with a silicon glass substrate 102, although othersubstrates can be used in different embodiments.

The process 200 begins at 202 in FIG. 3 with polishing of the top andbottom silicon substrate surfaces 102 a and 102 b of the substrate 102via corresponding polishing processes 302 and 304 as shown in FIG. 4.Any suitable polishing processing steps 302 and 304 can be used at 202in FIG. 3, whether concurrently as a single process or separately in anyorder to provide atomically smooth substrate surfaces 102 a and 102 b,where the polished top side 102 a is prepared via the process 302 foreventual wafer bonding to the upper silicon glass window 108 at 212 andthe process 303 provides a smooth bottom surface on the second side 102b suitable for wafer bonding to a lower silicon glass window 106 at 206.

At 204 in FIG. 3, the cavity or opening 103 is etched through the topand bottom of the substrate 102 using an anisotropic wet etch process306 using a mask 101 as shown in FIG. 5 to form sidewalls 104 at anangle θ. Any suitable etch process 306 may be used, where anisotropicetching advantageously provides angled sidewalls 104 as shown in FIG. 5.The etch process 306, moreover, provides a surface texture on thesidewalls 104 significantly rougher than the surface texture of thepolished first side 102 a of the substrate 102. As discussed furtherbelow, this texture differential between the substrate top side 102 aand the sidewalls 104 advantageously facilitates edge contact pinning tomitigate or avoid formation of crystallized alkali metal material on thecavity bottom 103 b of the finished vapor cell 100.

At 206 in FIG. 3, the first or lower window 106 is bonded to thesubstrate second side 102 b via a bonding process 308 as shown in FIG.6. The bonded lower window 106 can be any transparent materialcompatible with the bonding and UV exposure process steps, and forms thecavity bottom 103 b. Any suitable process 308 can be used at 206 forbonding the lower window structure 108 to the lower side 102 b of thesubstrate 102.

Referring also to a filling process 319 in FIGS. 7A and 7B, the cavity103 is entirely or substantially fully filled at 208 in FIG. 3 with analkali metal aqueous solution 112 such that the solution 112 extends toa contact line 112 c substantially at a top of the cavity sidewalls 104at or proximate to the first substrate side 102 a. Any suitable process310 can be used to dispense or otherwise introduce the solution 112 intothe cavity 103 as shown in FIG. 7A, such as pouring or pressurizedinjection. The inventors have appreciated that increasing the volume offluid 112 to entirely fill the cavity 103 advantageously increases thesurface area contact of the solution 112 to the sidewalls 104. Moreover,the sidewalls 104 have a fairly rough texture relative to the atomicallyflat glass at the top side 102 a of the substrate 102 due to the etchprocess 306 in FIG. 5 above, which enhances the strength of the fluidedge pinning at the contact line 112 c. In addition, the substantiallycomplete filling of the cavity 103 maximizes the distance between thepinned fluid interface at the line 112 c and the un-pinned interface atthe center of the fluid-air boundary of the solution 112. The cavity 103need not be entirely filled in all embodiments, wherein slightunderfilling is contemplated in certain embodiments.

The substantially complete cavity filling at 208 is in contrast toconventional alkali metal introduction in the fabrication of vaporcells, however, which typically involved introduction of material onlyat the bottom of a cavity. Any solution can be used having alkali metalin an aqueous form at 208, where the mixture process is adjusted in oneimplementation to target a desired final alkali metal mass for a givencell design. For example, the desired final Cs mass is set by themixture and amount of CsN₃ in one embodiment at a lower concentrationthan would be used using conventional dispensation approaches that didnot substantially fill the cavity.

Following the fluid introduction, the process 200 continues at 210 withdrying of the aqueous solution 112 at a controlled temperature andhumidity via a drying process 312 shown in FIGS. 8A, 9A, 10A, 11A, and12A to substantially maintain the edge contact pinning of the aqueoussolution 112 at a pinned fluid interface along the contact line 112 cbetween the aqueous solution 112 and the sidewalls 104. The inventorshave further appreciated that maintenance of the edge contact pinning ofthe aqueous solution 112 during the drying at 210 creates a preferencefor fluid loss through evaporation at and near the center of theun-pinned air-fluid interface due to increased stability at the pinnedinterface line 112 c and random-walk molecular mechanics. Thetemperature and humidity controlled drying process 312 providespreferential evaporation of the aqueous solution 112 proximate thecenter of an unpinned air-fluid interface as well as outward capillaryfluid flow of the aqueous solution 112 from the unpinned air-fluidinterface 112 c toward the pinned fluid interface at the contact line112 c. In addition, the process 312 promotes drying of dispersed aqueoussolution 112 to form crystallized alkali metal material 120 proximatethe sidewalls 104 and inhibits drying of dispersed aqueous solution 112on the cavity bottom 103 b as seen in FIGS. 8A-12B.

In certain embodiments, the drying process 312 involves controlling theevaporation rate of the aqueous solution 112 by temperature and relativehumidity control of the ambient process conditions in order to maintaina non-zero lateral temperature gradient in the aqueous solution 112,where the highest temperatures are near the sidewalls 104. The fluidtemperature gradient, in turn, promotes outward capillary flow of theaqueous solution 112 toward the sidewalls 104 to mitigate or avoidformation of crystallized alkali metal material 120 on the cavity bottom103 b. Unlike conventional solution drying approaches, the disclosedprocess 200 provides a mechanism for moving the material toward thesidewalls 104 and thus away from the cavity bottom 103 b to facilitatean obstruction-free optical path in the finished vapor cell 100. As seenin the drying process progression of FIGS. 8A-12B, the solution 112initially fills the entire cavity 103 (FIGS. 8A and 8B) with the process312 enhancing evaporation upward from the center, shown at arrow 116 inFIG. 8A, with the lower liquid evaporation from the cavity edges alongthe contact line 112 c being replenished by capillary outward liquidflow from the cavity interior as depicted by the dashed arrows in FIG.8A and the outward arrows shown in FIG. 8B, thus further maintaining thecontact pinning to the sidewalls 104.

As the drying process 312 continues in FIGS. 9A and 9B, the preferentialevaporation from the center at 116 causes a drop in the fluid levelwhile the outward edges of the fluid 112 remain pinned at the sidewalls104 along the contact line 112 c through molecular pinning forces to therough sidewall surfaces and fluid replenishment aided by the thermalgradient. As the temperature of the fluid 112 decreases from thesidewall edges 104 to the center at arrow 116, the solvation limits forsolvated CsN₃ also decrease, so for the CsN₃ species to remain in itsstable solvated state as a fluid solution 112, the process 312 providesa preferential migration via capillary fluid flow outward toward thewarmer temperatures to at the sidewalls 104. This outward fluidmigration allows the solution 112 to remain fully solvated until thecapillary flow during the final stages of the solution drying can becomethe driving factor as shown in the continued drying processing in FIGS.10A and 10B (further center fluid level drop).

This continues until the center reaches the cavity bottom 103 b,exposing a central portion of the lower glass window 106 as shown inFIGS. 11A and 11B. It is noted that the process 200 advantageouslyremoves all or substantially all the fluid material from the center andthe reduced fluid volume begins to reduce the vertical location of thepinned contact line 112 c along the sidewalls 104. As seen in FIGS. 11Aand 11B, the upper portions of the sidewalls 104 include dried dispersedaqueous solution material 120 (e.g., CsN₃) formed as a ring or bandaround the tops of the sidewalls 104. Suitable processes 312 can resultin the initial exposure of the cavity bottom 103 b either before, orafter, or concurrently with, the initial downward movement of the edgepinning contact line 112 c from the tops of the sidewalls 104.

The drying process 312 is continued at 210 in FIG. 3 until the solution112 is completely dried as shown in FIGS. 12A and 12B, where the driedmaterial 120 (e.g., crystallized alkali metal material such as CsN₃) isformed substantially exclusively along the sidewalls 104, and little orno alkali material is found along the cavity bottom 103 b. Thus, even ifa certain amount of dried material 120 is found along the outer edges ofthe cavity bottom 103 b (e.g., on the upper side of the lower glasswindow 106), CsN₃ solids 120 are localized to the vapor cell perimeterin the illustrated embodiment, outside the vapor cell active area usedfor an unobstructed optical channel.

The inventors have appreciated that complete or substantially completecavity filling at 208 and control of the evaporation rate through thecontrolled temperature and relative humidity in the process 312 at 210in FIG. 3 facilitates these transfer mechanisms to enhance thedeposition of dried material 120 along the sidewalls 104, whilemitigating or eliminating location of dried material 120 on the cavitybottom 103 b. Suitable drying processes can be determined by evaporationrate control to advantageously balance the preferential centerevaporation rate and thermal gradient-driven outward capillary flow fora given cell design in consideration of volume, cavity dimensions,aqueous solution mixture and other process and design variables.

In practice, the inventors have appreciated that excessive evaporationrates may inhibit the capacity for CsN₃ to remain sufficiently solvateduntil the capillary flow pulls the remaining fluid to the cell sidewalls104, whereas too slow an evaporation rate reduces the temperaturegradient leading to reduced localization of solid CsN₃ at the pinnedinterface. Non-limiting examples for drying CsN₃ include control of theprocess 312 at 46.1% relative humidity at 20.9° C. to form a generallyopen cavity bottom 103 c, 48.1% relative humidity at 20.9° C. for aclear cavity bottom 103 c with a visible ring of solid dried material120 around the sidewall edge, 49% relative humidity at 20.5° C. for arelatively clear cell bottom 103 c with some solid material formation120, and 51.8% relative humidity at 20.9° C., with higher relativehumidity settings at 51.8% at 20.9° C. resulting in a relatively clearso bottom 103 c with sublimation occurring in the active cell area.Further increased relative humidity of 55.6% at 20.7° C. results insolid formation in the center of the cell bottom 103 c, and still slowerevaporation rates using a 60.8% relative humidity at 20.3° C. yields ahazy cell window with large amounts of sublimation of solid material 120on the cell bottom 103 c. Other suitable process conditions can be foundfor facilitating replenishment of liquid evaporating at 210 in FIG. 3from the reservoir sidewall edges 104 by liquid from the reservoirinterior through control of the preferential center evaporation rate togenerate suitable thermal gradients during the drying process 312, wherethe resulting outward capillary flow can carry virtually all thedispersed material 120 within the cavity 103 to the sidewalls 104 beforesolidification.

Continuing at 212 in FIG. 3, a transparent upper window 108 such assilicon glass is bonded to the first side 102 a of the substrate 102 viaa bonding process 314 as shown in FIGS. 13A and 13B in order to seal thecavity 103 using any suitable bonding technique, and the crystallizedalkali metal material 120 is exposed to UV photolysis process 316 at 214to form an alkali metal gas (e.g., Cs) in the sealed cavity 103, withcrystallized alkali metal material 105 remaining on the sidewalls 104 asshown in FIGS. 14A and 14B. In a given process for concurrentfabrication of many cells 100, the products 100 are separated at 216into individual dice to complete the process 200, leaving a vapor cell100 as shown in FIGS. 1 and 2. As discussed above, the process 200advantageously provides a novel, low-cost, method for fabrication ofhigh-density Cs vapor cells 100 with complete filling and controlleddrying facilitating self-assembly of CsN₃ solids along the perimetersidewalls 104 outside the active area of a vapor cell 104 on a scalewhere direct manipulation is not possible. Other higher-cost solutionsare available, but these require use of two inter-connected cells, oneas CsN₃ reservoir cell, and one as laser interrogation cell, but thisinvolves additional fabrication cost and complexity as well assignificant increase in product size and material costs.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In addition, although a particular feature of thedisclosure may have been disclosed with respect to only one of multipleimplementations, such feature may be combined with one or more otherfeatures of other embodiments as may be desired and advantageous for anygiven or particular application. Also, to the extent that the terms“including”, “includes”, “having”, “has”, “with”, or variants thereofare used in the detailed description and/or in the claims, such termsare intended to be inclusive in a manner similar to the term“comprising”.

What is claimed is:
 1. A method for fabricating a vapor cell,comprising: forming a cavity including at least one sidewall extendingbetween first and second sides of a substrate; bonding a lower window tothe second side of the substrate to form a cavity bottom; filling thecavity with an aqueous solution comprising an alkali metal material, theaqueous solution extending to a contact line substantially at a top ofthe at least one sidewall of the cavity proximate the first side of thesubstrate; drying the aqueous solution at a controlled temperature andhumidity to substantially maintain edge contact pinning of the aqueoussolution at a pinned fluid interface between the aqueous solution andthe at least one sidewall to promote preferential evaporation of theaqueous solution proximate a center of an unpinned air-fluid interfaceand outward capillary fluid flow of the aqueous solution from theunpinned air-fluid interface toward the pinned fluid interface and topromote drying of dispersed aqueous solution to form crystallized alkalimetal material proximate the at least one sidewall and to inhibit dryingof dispersed aqueous solution on the cavity bottom; bonding an upperwindow to the first side of the substrate to seal the cavity; andexposing the crystallized alkali metal material to UV photolysis to forman alkali metal gas in the sealed cavity.
 2. The method of claim 1,wherein drying the aqueous solution comprises controlling an evaporationrate of the aqueous solution to maintain a non-zero lateral temperaturegradient in the aqueous solution with highest temperatures proximate theat least one sidewall to promote outward capillary flow of the aqueoussolution toward the at least one sidewall to mitigate formation ofcrystallized alkali metal material on the cavity bottom.
 3. The methodof claim 2, wherein forming the cavity comprises forming a unitarycavity separated by the at least one sidewall from any other cavityformed in the substrate.
 4. The method of claim 2, wherein forming thecavity comprises performing an etch process to provide a surface textureon the at least one sidewall significantly rougher than a surfacetexture of the first side of the substrate.
 5. The method of claim 4,wherein forming the cavity comprises performing an anisotropic etchprocess to form the at least one sidewall at a non-zero angle.
 6. Themethod of claim 2, wherein the alkali metal material is Cesium Azide(CsN₃).
 7. The method of claim 2, wherein forming the cavity comprisesperforming an anisotropic etch process to form the at least one sidewallat a non-zero angle.
 8. The method of claim 1, wherein forming thecavity comprises forming a unitary cavity separated by the at least onesidewall from any other cavity formed in the substrate.
 9. The method ofclaim 1, wherein forming the cavity comprises performing an etch processto provide a surface texture on the at least one sidewall significantlyrougher than a surface texture of the first side of the substrate. 10.The method of claim 9, wherein forming the cavity comprises performingan anisotropic etch process to form the at least one sidewall at anon-zero angle.
 11. The method of claim 1, wherein forming the cavitycomprises performing an anisotropic etch process to form the at leastone sidewall at a non-zero angle.
 12. The method of claim 1, wherein thealkali metal material is Cesium Azide (CsN₃).
 13. A vapor cell,comprising: a substrate structure comprising: a first side, a secondside, a cavity defined by at least one sidewall extending between afirst opening in the first side and a second opening in the second side,and a crystallized alkali metal material formed on at least a portion ofthe at least one sidewall; a lower window engaging the second side ofthe substrate to form a transparent cavity bottom substantially free ofcrystallized alkali metal material; an upper window engaging the firstside of the substrate to seal the cavity and form a transparent cavitytop; and an alkali metal gas in the sealed cavity.
 14. The vapor cell ofclaim 13, wherein the at least one sidewall comprises a surface texturesignificantly rougher than a surface texture of the first side of thesubstrate.
 15. The vapor cell of claim 14, wherein the at least onesidewall is at a non-zero angle relative to an axis extending betweenthe first and second sides.
 16. The vapor cell of claim 14, whereinsubstrate structure comprises only one cavity.
 17. The vapor cell ofclaim 13, wherein the at least one sidewall is at a non-zero anglerelative to an axis extending between the first and second sides. 18.The vapor cell of claim 17, wherein substrate structure comprises onlyone cavity.
 19. The vapor cell of claim 13, wherein substrate structurecomprises only one cavity.
 20. A vapor cell, comprising: a siliconsubstrate, including a top side, a bottom side, and a single openinghaving at least one angled sidewall extending from the top side to thebottom side to define a cavity; a substantially transparent top windowcovering the single opening at the top side; a substantially transparentbottom window covering the single opening at the bottom side and beingsubstantially free of crystallized alkali metal material, the top andbottom windows sealing the cavity and allowing light to pass through thecavity; and a Cesium gas in the sealed cavity.